Photosensitive material and process of making same

ABSTRACT

A photosensitive material, suitable for image-wise recording, e.g., holographic recording of data or other information, comprises an organic species in an organic-inorganic matrix, the organic species comprising a material having a refractive index which changes upon exposure to actinic radiation. The organic-inorganic matrix may be an organically modified glass. The photo-sensitive material may be made using the sol-gel process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional patentapplication No. 60/363,038 filed Mar. 11, 2002, the contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to photosensitive materials and processes formaking same and is especially, but not exclusively, applicable tophotosensitive materials suitable for image-wise recording, for exampleholographic recording of data or volume holograms.

2. Background Art

There are many substances that are known to be photosensitive andsuitable for holographic image recording. Those include silver halides,photopolymers, dichromated gelatins, and photorefractive, ferromagnetic,photochromic, photodichroic and azo-dye materials. In these materials,temporary or permanent changes of real or imaginary part of refractiveindex are induced upon-illumination of the material by actinic radiationand, typically, subsequent processing steps are required.

For high-resolution phase imaging such as that required in practicalholographic data storage devices, there are several properties that anoptimum recording material should exhibit. These include high dynamicrange and sensitivity, high resolution, negligible shrinkage uponholographic exposure, low noise, phase image forming mechanism, imageand dimensional stability, and fabricability into thick films andmonoliths, to name the most relevant ones. However, to meet all theseproperties in a single material is a significant challenge that has notbeen satisfactorily overcome up to present and lack of such medium iscurrently considered in the art as one of the main hurdles indevelopment of practical holographic data storage systems. Acomprehensive review of holographic recording materials for data storageapplications can be found in Holographic recording materials for opticaldata storage, by P. Cheben, in Advanced Optics, Chapter 1, pp. 565–609(Ariel, Barcelona, 2002).

Existing Techniques

Photopolymers are believed to be promising holographic recordingmaterials for applications such as display or security holograms,Write-Once Read-Many times (WORM) holographic memories, or HolographicOptical Elements (HOEs). A number of photopolymerizable systems havebeen developed up to present, such as those disclosed in U.S. Pat. Nos.3,658,526; 4,942,112; 4,959,284 and 4,994,347 assigned to E. I. Du Pontde Nemours and Company, U.S. Pat. Nos. 4,588,664; 4,696,876 and4,970,129 assigned to Polaroid Corporation, U.S. Pat. No. 3,993,485assigned to Bell Telephone Laboratories, U.S. Pat. No. 4,173,474assigned to Cannon Kabushiki Kaisha, and U.S. Pat. No. 3,694,218assigned to Hughes Aircraft Company. Another promising class ofmaterials are porous glasses impregnated by photopolymerizable materialssuch as those disclosed in Spanish patent P9700217 assigned to theInstituto Nacional de Tecnica Aeroespacial, Japanese patents JP6148880assigned to Nippon Sheet Glass Co. Ltd. and JP61141476A assigned to SonyCorp.

Limitations or Drawbacks of Existing Techniques

Application of photopolymers to volume holography and data storage isseverely limited due to their limited thickness, high shrinkage duringholographic exposure and need for solvent processing. A recording mediumof millimeter thickness or more and exhibiting high photoinducedrefractive index change is required to achieve high storage density byrecording multiple volume holograms, separated from each other by theBragg effect, in the same spatial location.

The most important limitations are imposed by organic polymeric bindersthat limit thickness of the medium, usually to less than a few hundredmicrons, and temperature- and light-induced dimensional changes that candistort the holograms and degrade the fidelity with which the storedimages can be retrieved. An approach to prepare thicker photopolymers isto use resins consisting of two independent photopolymerisable systemssuch as those disclosed in L. Dhar et al., Opt. Lett. 24, pp. 487–489(1999), , in which matrix-forming oligomers are first precured to a gelstate and then the hologram is recorded by photoinduced polymerizationof monomers dissolved in the resin. However, this approach does notsimultaneously achieve high dimensional stability and maximumphotoinduced refractive index change. To increase the rigidity of thematerial, higher levels of precuring are required, which decreasesdiffusional mobility of the monomer in the resin and degradesholographic properties of the photopolymer. In addition, some monomericspecies are inevitably consumed (polymerized) during the precuring step,which in turn reduces dynamic range of the holographic recording and,ultimately, limits the data storage capacity of the material.

Porous glasses such as those fabricated by sol-gel technique appear tobe promising for data storage as they can typically be fabricated asbulk monoliths. However, the materials disclosed up to present,including Spanish patent P9700217 assigned to the Instituto Nacional deTecnica Aeroespacial, Japanese patents JP6148880A assigned to NipponSheet Glass Co. Ltd. and JP61141476A assigned to Sony Corp. suffer fromdrawbacks ranging from insufficient sensitivity and dynamic range forpractical applications to insufficient optical quality and thickness.

SUMMARY OF THE INVENTION

The present invention seeks to at least ameliorate the disadvantages ofsuch known materials, or at least provide an alternative.

According to the present invention, a photosensitive material comprisesa host matrix formed by an inorganic network and an organic-inorganicnetwork interpenetrating each other, the host matrix containing at leastone organic species having a refractive index which changes uponexposure to actinic radiation.

The organic species may comprise an efficient organic photosensitive andphotoinitiating species together with a monomer or a mixture ofmonomers.

The organic-inorganic matrix may comprise an organically modified glassyhost with the organic species dispersed therein and/or chemically-bondedthereto. The organically modified host of embodiments of the presentinvention may provide desired optical quality and physical and chemicalstability.

The organic species may be bonded to the organic-inorganic matrix bycovalent bonding.

Alternatively, the organic species may be a dispersion in theorganic-inorganic host.

The at least one organic species can be impregnated or entrapped as aguest within organic-inorganic matrix.

According to a second aspect of the invention, there is provided aprocess of making a photosensitive material comprising the steps offorming a host matrix comprising an inorganic network and anorganic-inorganic network interpenetrating each other, the host matrixcontaining at least one organic species having a refractive index thatchanges on exposure to actinic radiation.

Preferably, in embodiments of either aspect of the invention, thephotosensitive material is formed using a sol-gel process.

The preferred manufacturing process, namely the sol-gel technique, iswell known in the art, and has been widely used in the preparation ofoxide glasses by hydrolysis and condensation of metal alkoxides sincethe first synthesis of silica from silicon alkoxide was reported byEbelmen in 1844. Given that the sol-gel process can take place at roomtemperature, it may be used to incorporate an organic species, e.g.,organic molecules with low thermal stability, into an inorganic ororganic-inorganic matrix, e.g., a glassy host.

The organic species may be dispersed in an organically-modified hostover domains of several nanometers, which results in a materialexhibiting very low optical scattering, even if the refractive indicesof the organic species differ from that of the host matrix. The hostmatrix is a crucial component affecting physical properties of thephotosensitive material, such as its rigidity, resistance to cracking,environmental stability, reduction of dimensional changes uponholographic exposure, and maximum achievable thickness. It also affectsoptical properties of the composite, including bulk refractive index,optical homogeneity and photoinduced refractive index change.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription, in conjunction with the accompanying drawings, of preferredembodiments of the invention, which are described by way of exampleonly.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates geometry used for glass characterization byholographic grating recording;

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the invention disclosed herein include a photosensitivematerial and a process of fabricating same that provide a highresolution and stable phase image recording properties. Using thesol-gel process, the material may be prepared by incorporatingphotosensitive and photopolymerizable organic species, as dopants, intoa solution of one or more liquid glass precursors and the lattertransformed via controlled hydrolysis and polycondensation reactionsinto a solid material of a high optical quality. High optical qualityand excellent imaging properties such as high dynamic range,sensitivity, resolution and image stability may be achieved by acombination of efficient photosensitive and phototopolymerizable specieswith an organically modified host matrix.

The organic-inorganic matrix may comprise a hybrid network synthesizedby using organo alkoxysilanes as one or more of the precursors for thesol-gel reaction in which organic groups are introduced within aninorganic network through the Si—C-bond. Alternatively, theorganic-inorganic matrix can be formed via the co-condensation offunctionalized oligomers or polymers with metal alkoxides in whichchemical bonding is established between inorganic and organic phases.

The organic-inorganic matrix can also be synthesized through theformation of inorganic species within a polymer matrix. Specifically,inorganic species, generally in the form of particles with acharacteristic size of a few hundred angstroms, can be generated in situwithin the polymer matrix by first swelling cross-linked, ionomeric, orcrystalline polymeric host with a compatible solution containing metalalkoxides followed by the promotion of the sol-gel reaction of theinorganic.

Yet another alternative would be to obtain the organic-inorganic matrixby either the infiltration of previously formed oxide gels withpolymerizable organic monomers or the mixing of polymers with metalalkoxides in a common solvent.

The matrix can also be formed by interpenetrating networks andsimultaneous formation of inorganic and organic phases. By usingtriethoxysliane R′Si(OR)₃ or diethoxysilanes R′R″Si(OR)₂ as theprecursor with R′ and R″ being a polymerizable group such as an epoxygroup, an organic network can be formed within the inorganic network byeither photochemical or thermal curing of such groups.

Yet another process is to form the matrix from inorganic/organicsimultaneous interpenetrating networks, where both inorganic glass andpolymer formation occur concurrently. Such transparent composites may besynthesized through a synchronous application of the aqueousring-opening metathesis polymerization of cyclic alkenyl monomers andthe hydrolysis and condensation of metal alkoxides.

Other ways of making the matrix include employing polymerizable monomersas the cosolvents such that all the components (i.e. the monomers andsol-gel precursors) contribute either to the inorganic matrix or to theorganic polymer matrix, which together form the organic-inorganicmatrix, to avoid large scale shrinkage.

In a preferred embodiment of this invention, the organic-inorganicmatrix is achieved by copolymerization of epoxysilanes such as(3-glycidoxypropyl)trialkoxysilane with a tetraalkoxysilane in thepresence of dispersed photosensitive, photoinitiating andphotopolymerizable species.

The organic-inorganic matrix is a crucial component affecting physicalproperties of the photosensitive material, such as its rigidity,resistance to cracking, environmental stability, elimination ofdimensional changes upon holographic exposure, maximum achievablethickness, and optical properties of the composite, including bulkrefractive index, optical homogeneity and photoinduced refractive indexchange.

A wide variety of free radical or cationic initiators can be used toinitiate polymerization while the photoactivation of initiator moleculescan be achieved by a number of sensitizing molecules well known in theart. A review of free-radical photoinitiators can be found for examplein Photoinitiators for free-radical-initiated photoimaging systems, byB. M. Monroe and G. C. Weed, Chemical Reviews, vol. 93, pp. 435–448,(1993). A large number of sensitizers and initiators providing blue,green, red and near infra-red sensitivity can be used in theimplementations of this invention, including halogen-substitutedacetophenones, chromophore-substituted triazines, azo dies, benzoinethers, ketals, o-acylated oximino ketones, acylphosphine oxides,aromatic ketones, hexaarylbisimidazoles,bis(p-dialkylaminobenzilidene)ketones, thioxanthones, ketocoumarins,9-phenylacridine, die-sensitized systems such as xantene, acridinium;phenazine and thiazine dyes in combination with activators such asamines, sulfinates, enolates, carboxylates and organotine compounds,dye-borate complexes, ferrocenium salts, aluminate complexes, and proticacid generators.

Preferred sensitizers and initiators for use in embodiments of thisinvention include organometallic systems such asdicyclopentadienyltitanocenes, in particularbis(pentafluorophenyl)titanocene, titanocene/N-phenylglycine, andbis(μ⁵-2,4-cyclopentadien-1-yl)-bis-[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium;and bis(p-dialkylaminobenzilidene)ketones in combination withhexaarylbisimidazole initiating systems such as2,2′,4,4′,5,5′-hexaarylbisimidazole with charge transfer agents such as2-mercaptobenzoxazole. Concentration of the sensitizer and initiator inthe matrix is from 0.001 to 10%, preferably from 0.01 to 5% depending onthickness of the material.

A wide variety of ethylenically unsaturated monomers or oligomerscapable of free radical addition polymerization can be used inembodiments of this invention. Acrylate and methacrylate monomers arepreferred because of their high propagation and low termination rates.Liquid monomers are preferred, but solid monomers can be used eitheralone or in combination with a liquid monomer, provided that solidmonomer is soluble in the sol-gel matrix. Furthermore, monomers withhigh molar refraction are preferred as they maximize the refractiveindex change per one diffusing monomer molecule upon holographicexposure. Advantageously, the monomers have good solubility in silicasol without degrading the optical and mechanical properties of thecomposite. Suitable monomers include but are not limited to phenylacrylate, 2-phenoxyethyl acrylate, N-vinylcarbazol, isobornyl acrylate,3,6-dibromo-9-vinyl carbazol, p-chlorophenyl acrylate, hexanedioldiacrylate, vinyl benzoate, tert-butyl hydroperoxide, hexanedioldiacrylate, 2,4,6-tribromophenyl acrylate, phenyl acrylate,orthobiphenyl acrylate, orthobiphenyl methacrylate, di(2-acryloxyethyl)ether of bisphenol-A, 2-phenylethyl acrylate, di-(p-clorophenoxy)ethylacrylate, and pentachlorophenyl acrylate. Other suitable monomers aremultifunctional monomers containing two or more ethylenicallyunsaturated groups. Monomers of this type include but are not limited toethylene glycol diacrylate, diethylen glycol diacrylate, 1,4-butanedioldiacrylate, decamethylene glycon diacrylate, 1,4-cycolhexanedioldiacrylate, glycerol diacrylate, glycerol triacrylate, ethylene glycoldimethacrylate, butylene glycol dimethacylate, tripropylene glycoldiacrylate, di(2-acryloxyethyl) ether of bisphenol-A,di(2-acryloxyethyl) ether of tetrabromo-bisphenol-A. Other suitablemonomers are those capable of cationic ring opening polymerization(CROP). Such CROP monomers have two or more cyclohexene oxide groupslinked through siloxane chain segments, for example such as1,3-bis[2-(3{7-oxabicyclo[4.1.0]heptyl})ethyl]-tetramethyl disiloxane.Polymerization of such monomers can be initiated by strong protic acidsthat are generated during the photo decomposition of, for example,sulfonim or iodonium salts. Concentration of the monomers in the glassis from 5 to 70%, preferably 20–50%.

General Procedures

Sensitizing and photoinitiating species and monomer were added intosol-gel starting solution containing one or more hydrolyzed sol-gelprecursors and an acidic catalyst, prior to the gelation point. Aftervigorously stirring the solution for about 20 minutes at about 40° C., ahomogeneous solution was obtained. It was then cast into Teflon or glassvials and allowed to gel and dry for about 10 days at about 60° C. Drymonoliths were polished to optical-grade slabs of thickness up toseveral mm. Films of thickness up to 500

m were prepared by casting the solution onto a borosilicate glass plate.The films were dried for about 5 days at about 60° C.

The given preparation times and temperature are typical for the examplesprovided in this invention but are not to be taken by way of limitation;rather they can be varied as is well known in the art.

To characterize holographic performance of the glass, plane gratingswere recorded in the glass by two mutually coherent plane-wave writingbeams 1 and 2, as illustrated in FIG. 1. The writing beams of wavelength514.5 or 488 nm, were from an Ar-ion laser operating in a singlelongitudinal and transversal mode. The grating growth was studied duringand after the holographic exposure by diffraction of read-out beam 3.The angle of incidence of the read-out beam was set to satisfy thefirst-order Bragg condition at which the intensity of the first-orderdiffracted beam (beam 4) and thus, diffraction efficiency η, ismaximized. To minimize a possible photochemical influence of theread-out beam, a low-intensity (0.8 mW/cm²) beam of 632.8 nm wavelength(from a He—Ne laser) at which the absorption of the photoinitiator islow, was used. In the experiments, polarization of the writing andread-out beams was perpendicular (‘s’ polarization) to the plane definedby the sample normal and wave vectors of the writing beams, but ‘p’polarization can also be used.

After the holographic exposure, the samples were kept in the dark for 15minutes to allow any dark reactions to subside. The photoreaction wasthen completed by uniformly exposing the sample 5 minutes to UV light(wavelength 254 nm from a low-pressure mercury vapor lamp. Thediffraction efficiency dependence on the angle of incidence was measuredby detuning from the Bragg condition. This measurement yields thegrating angular selectivity □ defined as full-width at half-maximum(FWHM) of the angular selectivity curve. The refractive index modulationamplitude of the grating was calculated from the diffraction efficiencyand angular selectivity data using Kogelnik's coupled-wave theory (H.Kogelnik, Bell Syst. Tech. J. 48, p. 2909, 1969).

The mechanism of grating formation can be explained by the modelsdeveloped for holographic photopolymers (W. S. Colburn, Appl. Opt. 10,p. 1636, 1971; G. Zhao and P. Mouroulis, J. Mod. Opt. 41, p. 1929,1994). The monomer polymerization starts and proceeds preferentially inthe light regions of the illumination pattern. The depletion of themonomer in the light regions sets-up a spatial concentration gradientresulting in a diffusional flux of monomer molecules from the dark tothe light regions, along the grating vector. In addition, it is wellknown in the art that acrylic monomers shrink on polymerization. As themonomer molecules, separated at van der Walls distances, are convertedto covalently bonded polymer, voids are created in the light regions,and the resulting capillary forces draw the fresh monomer from the darkto the light regions. This process continues until either there is nomore unreacted monomer in the dark regions or transport of monomer ishindered by increased rigidity of the polymer in the polymerizedregions. Ultimately, permanent compositional and density changes andassociated refractive index modulation are induced in the material. Itis known that the diffusion of monomer is a critical mechanism in theformation of holographic gratings in photopolymerizable materials. Aporous matrix advantageously provides for efficient liquid monomertransport.

Embodiments of the present invention provide a new class ofphotosensitive material providing a significant advance in the field ofresearch on holographic recording materials. The material hasunprecedented refractive index modulation capability for a thickphotopolymerizable material. The holograms stored in the material arepermanent and stable; the constancy of Δn over a period of 12 months atroom temperature having been demonstrated. An additional advantage ofthe material, as compared to holographic photopolymers, is that it canbe easily fabricated into samples of high optical quality and thicknessof several millimetres. The failure of the prior art attempts to makephotopolymers of such a thickness with required holographic propertieshas been considered the main obstacle in development of holographicwrite-once memories. In contrast, photosensitive materials embodying thepresent invention exhibit both excellent holographic properties and easeof fabrication into crack-free thick films or monoliths of high opticalquality, as required for holographic storage applications.

Advantageously, characteristics required for volume holographicrecording, including photosensitivity, high refractive index modulation,desired material thickness, high optical quality, and dimensional andmechanical stability, are achieved by embodiments of the invention whichemploy a combination of an organic species, for example an organicphotosensitive, photoinitiating and monomeric species, within anorganic-inorganic matrix, for example organically-modified, glass. Afurther advantage is that photosensitive materials embodying the presentinvention can be polished to optical-grade.

Although the above-described preferred embodiments of the inventionrefer to phase holographic recording, the invention is applicable toamplitude holographic recording and, indeed, other forms of image-wiserefractive index and/or absorption recording.

The preferred manufacturing process, namely the sol-gel technique, thatcan take place at room temperature, may be used to incorporate anorganic species, e.g. organic molecules with low thermal stability, intoan organic-inorganic matrix, e.g., organically modified glass.

EXAMPLES

Samples 1–2.

-   -   Organic-inorganic matrix: copolymerized        3-glycidoxypropyltrimethoxysilane and tetramethyl orthosilicate,        1350 mg,    -   Monomer: phenyl acrylate, 830 mg,    -   Initiator:        bis(μ⁵-2,4-cyclopentadien-1-yl)-bis-[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium,        21 mg.        Sample 3.    -   Organic-inorganic matrix: copolymerized        3-glycidoxypropyltriethoxysilane and tetraethyl orthosilicate,        1350 mg,    -   Monomers: 2-phenoxyethyl acrylate, 580 mg, and N-vinylcarbazol,        40 mg,    -   Initiator:        bis(μ⁵-2,4-cyclopentadien-1-yl)-bis-[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium,        15 mg.        Sample 4.    -   Organic-inorganic matrix: copolymerized        3-glycidoxypropyltriethoxysilane, tetraethyl orthosilicate, and        methacryloxypropyl triethoxysilane, 1350 mg,    -   Monomers: isobornyl acrylate, 750 mg,    -   Initiator system:        2,5-bis[[(4-diethylamino)phenyl]methylene]cyclopentanone, 0.4        mg, hexaarylbiimidazole, 40 mg, and 2-mercaptobenzoxazole, 50        mg.        Holographic properties of these compositions are summarized in        Table 1.

Sample number T [μm] SF [lp/mm] P [mW/cm²] λ[nm] Δn 1 270 350 90 514.54.6 · 10⁻³ 2 320 1230 70 514.5 4.4 · 10⁻³ 3 1100 350 80 514.5 1.4 · 10⁻³4 260 350 70 488   3 · 10⁻³Where: T is the sample thickness, SF the grating spatial frequency, Pthe total power in both recording beams, λ the wavelength of actinicradiation, and Δn the measured refractive index modulation of thegrating.

Although embodiments of the invention have been described andillustrated in detail, it is to be clearly understood that the same isby way of illustration and example only and not to be taken by way ofthe limitation, the spirit and scope of the present invention beinglimited only by the appended claims.

1. A photosensitive material comprising a host matrix formed by aninorganic network and an organic-inorganic net-work interpenetratingeach other, the host matrix containing at least one organic specieshaving a refractive index which changes upon exposure to actinicradiation.
 2. A photosensitive material according to claim 1, whereinthe organic species comprises one or more of efficient organicphotosensitive and photoinitiating species together with a monomer or amixture of monomers and the host matrix comprises interpenetratinginorganic and organic-inorganic networks with the organic speciesdispersed therein or chemically-bonded thereto, or both dispersedtherein and chemically-bonded thereto.
 3. A photosensitive materialaccording to claim 1, wherein the photosensitive material comprises aproduct of a sol-gel process.
 4. A photosensitive material according toclaim 1, wherein the organic species is selected from the groupcomprising halogen-substituted acetophenones, chromophore-substitutedtriazines, azo dies, benzoin ethers, ketals, o-acylated oximino ketones,acyl phosphine oxides, aromatic ketones, hexaarylbisimidazoles,bis(p-dialkylaminobenzilidene) ketones, thioxanthones, ketocoumarins,9-phenylacridine, die-sensitized systems such as xantene, acridinium,phenazine and thiazine dyes in combination with activators such asamines, sulfinates, enolates, carboxylates and organotine compounds,dye-borate complexes, ferrocenium salts, aluminate complexes, proticacid generators such as sulfonium or iodonium salts capable ofinitiating cationic polymerization, and organometallic systems such asdicyclopentadienyltitanocenes, in particularbis(pentafluorophenyl)titanocene, titanocene/N-phenylglycine, andbis(⁵-2,4-cyclopentadien-1-yl)-bis-[2,6-difluoro-3-(1H-pyrrol-1-yt)phenyl]titanium;and bis(p-dialkylaminobenzilidene) ketones in combination with ahexaarylbisimidazole initiating system with charge transfer agents suchas 2-mercaptobenzoxazole.
 5. A photosensitive material according toclaim 1, wherein the organic species is selected from the monomerscapable of cationic polymerization and ethylenically unsaturatedmonomers capable of free radical addition polymerization.
 6. Aphotosensitive material according to claim 5, wherein the monomers areselected from the group comprising phenyl acrylate, 2-phenoxyethylacrylate, N-vinylcarbazol, 3,6-dibromo-9-vinyl carbazol, p-chlorophenylacrylate, hexanediol diacrylate, vinyl benzoate, tert-butylhydroperoxide, hexanediol diacrylate, 2,4,6-tribromophenyl acrylate,phenyl acrylate, orthobiphenyl acrylate, orthobiphenyl methacrylate,di(2-acryloxyethyl) ether of bisphenol-A, 2-phenylethyl acrylate,di-(p-clorophenoxy)ethyl acrylate, pentachlorophenyl acrylate, ethyleneglycol diacrylate, diethylen glycol diacrylate, 1,4-butanedioldiacrylate, decamethylene glycon diacrylate, 1,4-cyclohexanedioldiacrylate, glycerol diacrylate, glycerol triacrylate, ethylene glycoldilmethacrylate, butylene glycol dimethacylate, tripropylene glycoldiacrylate, di(2-acryloxyethyl) ether of bisphenol-A,di(2-acryloxyethyl) ether of tetrabromo-bisphenol-A, and monomers thathave two or more cyclohexene oxide groups linked through siloxane chainsegments, including1,3-bis[2-(3{7-oxabicyclo[4.1.0]heptyl})ethyl]-tetramethyl disiloxane.7. A material according to claim 1, wherein the organic-inorganicnetwork comprises a material synthesized using organo alkoxysilanes asone or more of the precursors for a sol-gel reaction in which organicgroups are introduced within an inorganic network through the≡Si—C—″bond.
 8. A material according to claim 1, wherein the host matrixmaterial comprises, in the presence of dispersed photosensitive,photoinitiating and photopolymerizable species, interpenetratingnetworks obtained by copolymerization of an epoxysilane and either orboth of a tetraalkoxysilane and a trialkoxysilane.
 9. A materialaccording to claim 8, wherein the epoxysilane is a(3-glycidoxypropyl)-trialkoxysilane.
 10. A material according to claim1, wherein the matrix comprises a said organic-inorganic network formedwithin said inorganic network by either photochemical or thermal curingthereof using a tetraalkoxysilane (Si(OR)₄) and either or both oftrialkoxysilane R′Si(OR)₃ and dialkoxysilanes R′R″Si(OR)₂ as theprecursor with R′ and R″ being a polymerizable group such as an epoxygroup.
 11. A material according to claim 1, wherein the matrix comprisesa material formed as simultaneous interpenetrating networks, where bothsaid inorganic network and said organic-inorganic network have beenformed concurrently.
 12. A process of making a photosensitive materialcomprising the steps of forming a host matrix comprising an inorganicnetwork and an organic-inorganic network interpenetrating each other,the host matrix containing at least one organic species having arefractive index that changes on exposure to actinic radiation.
 13. Aprocess according to claim 12, wherein the process comprises a sol-gelprocess.
 14. A process according to claim 12, wherein theorganic-inorganic network is synthesized using organo alkoxysilanes asone or more of the precursors for a sol-gel reaction in which organicgroups are introduced within an inorganic network through the≡Si—C—″bond.
 15. A process according to claim 12, wherein the matrixmaterial is formed by copolymerization of an epoxysilane and either orboth of a tetraalkoxysilane and a trialkoxysilane in the presence ofdispersed photosensitive, photoinitiating and photopolymerizablespecies.
 16. A process according to claim 15, wherein the epoxysilaneused is a (3-glycidoxypropyl) trialkoxysilane.
 17. A process accordingto claim 12, wherein the organic-inorganic network is formed as anorganically modified network within the inorganic network by eitherphotochemical or thermal curing thereof using a tetraalkoxysilane(Si(OR)₄) and either or both of trialkoxysilane R′Si(OR)₃ anddialkoxysilanes R′R″Si(OR)₂ as the precursor with R′ and R″ being apolymerizable group such as an epoxy group.
 18. A process according toclaim 12, wherein the matrix material is formed as simultaneousinterpenetrating networks, where both inorganic network andorganic-inorganic network formations occur concurrently.
 19. A processaccording to claim 12, comprising the step of employing polymerizablemonomers as the cosolvents such that all the components contributeeither to the inorganic network or to the organic polymer.
 20. Aphotosensitive material according to claim 1, wherein the host matrix isprepared by co-polymerization of sol-gel precursors of an inorganicnetwork and an organic-inorganic network.
 21. A material according toclaim 1, wherein the host matrix material comprises, in the presence ofdispersed photosensitive, photoinitiating and photopolymerizablespecies, interpenetrating networks obtained by copolymerization of atetraalkoxysilane and a trialkoxysilane.
 22. A process according toclaim 12, wherein the host matrix is prepared by copolymerization ofsol-gel precursors of an inorganic network and an organic-inorganicnetwork.
 23. A process according to claim 12, wherein the matrix isformed by copolymerization of a tetraalkoxysilane and a trialkoxysilanein the presence of dispersed photosensitive, photoinitiating andphotopolymerizable species.